Battle of Agincourt
The combatant armies were the English under Henry V and the French under Charles VI (although the King himself was incapacitated and actual command fell to the Constable, Charles d'Albret, and other French nobles).
The Hundred Years War between England and France had begun in 1337, and was concerned with the claim of the kings of England to the throne of France, based on the descent of the English king Edward III from the French Capetian king Philip IV, as against the French preference for the junior line from Philip's father, Philip III, through Charles of Valois, to Philip VI.
The 1415 campaign represented the renaissance of the claim, after a period of senescence under the victories of Charles V of France, and the trace of 1389, following which Henry IV of England had been unable to press any advantage he might have had whilst the Houses of Burgundy and Orleans fought over Charles VI's power.
Henry landed at Harfleur, which he besieged and took after 3 weeks, and, turning aside from the temptation of a march on Paris, raided across northern France towards Calais (which was then in English hands). The French, under d'Albret, successfully delayed him by using a feudal army to prevent his progress, but refusing to close for battle. Henry's army ran out of supplies, and d'Albret decided it was a fit time, with Autumn sweeping in and the weather promoting sickness among the English troops, to challenge. It is unknown whether he knew or not that Henry's army was chiefly longbowmen, with an armoured infantry corps at its core.
The English were advancing through a shallow valley between 2 woods, when the French closed the northern exit. This was 24 October, and the English were compelled to camp where they were, in rain, until the next day. They then deployed in a line of battle across the valley, probably men at arms and knights with groups of archers on either wing and in the centre. The French made three lines, or battles, with their cavalry on the wings, and their armoured infantry (many of them dismounted knights) in the centre. They had the disadvantage of thinking themselves superior (which in numbers they were), which led to the nobles competing for the "best" (ie most advanced) positions.
The weather was muddy, which hampered the side who moved. The first to do so were the English, but they 'dug in', with the archers putting down sharpened stakes, at extreme bowshot, and at the narrowest point between the woods, thereby securing their flanks. This compelled the French to attack, retreat or be cut down. Their cavalry attacked, but were beaten back, many of their horses being killed, and the stake-line preventing them getting to terms with the archers, who were firing on them. The infantry advanced, slowly, through the heavy soil, and were cut down by arrows, and those who survived and reached the English lines were disordered, tired, facing rested opponents, and unable to manoeuvre easily with comrades pressing in from behind them.
By weight of numbers they were able to force the English back a little and the second French battle was called in. But as they advanced the lightly-armoured English archers (who knew that, if France won, they would be slaughtered) picked up hatchets and other weapons and did bloody slaughter dodging in and out of the French combatants. The second French battle then met a like fate when they arrived. Many fled before they got there, seeing what had happened.
A French attack on the English baggage train led to the slaughter of the French prisoners (who had thought themselves safe as ransom), and the third French battle never fully entered the conflict. Then the fall of evening protected the English. In the battle France lost most of its fighting nobility and the initiative.
Afterwards Henry was able to make Calais, and to pursue the war the following year. In the longer term, his successes, headlined by Agincourt, won him the hand of the French princess Catherine of Valois as his Queen, and they married in 1420 CE.